I2C is a two-wire, serial data communication protocol. It is used in a wide array of applications including, but not limited to: reading and writing memory such as EEPROM memory, PCI card management, accessing low speed DAC and ADC, managing monitor or LCD settings, controlling speakers and audio systems, communicating over automotive communication networks, etc.
An integrated circuit (IC) that provides an I2C interface requires two pins dedicated to the I2C data signals. The I2C protocol requires an SCL signal that provides an I2C clock signal and an SDA signal that provides an I2C data signal. Thus, I2C requires two pins: one for the SCL signal and one for the SDA signal.
Some integrated circuits implement I2C communication with pins that are normally used for other functions by switching between I2C and another (e.g. parallel) communication mode, or between I2C and other functions associated with the integrated circuit. However, a circuit that switches between an I2C mode and a non I2C mode often requires an additional pin, or some other mechanism, to turn the I2C mode on and off.